The present invention relates to a reflector filter operating with surface acoustic waves, having an input transducer, an output transducer and having reflector structures which are effective connected in series for the path of surface acoustic waves between input transducer and output transducer and which deflect the direction of the acoustic waves several times between input transducer and output transducer.
U.S. Pat. Nos. 4,484,160 and 4,520,330 disclose a surface-wave reflector filter, known as RAC filter, having an input transducer, an output transducer and two reflector structures which, with reflector fingers positioned obliquely, face each other (lying next to one another) in such a way that an acoustic wave proceeding into one reflector is deflected by approximately 90.degree. and proceeds into the other reflector and is fed by the latter again with an approximately 90.degree. change of direction to the output transducer. A filter of this type is suitable for providing a time period. The finger structures of the aforesaid reflectors can be weighted and/or dispersive in accordance with the desired transmission function. In particular, finger displacement weighting, finger omission weighting and finger rotation can be provided as finger weighting. Such a known reflector filter has two acoustic tracks, namely one track with the input transducer and one reflector, and the second track extending parallel thereto with the other reflector and the output transducer. The tracks coincide with the respective main wave propagation direction of the transducer associated with the track in question.
The disadvantages of a reflector filter of this type are a certain temperature dependency of the angle accuracy of the reflection, to be precise even in the case of a substrate made of quartz, and a certain Mount of transmission losses. However, a reflector filter of this type requires only a minimum size substrate, since for reasons of stability the width of an oblong substrate plate in question here could not be made optimally small for example, for an in-line arrangement, taking only the space requirement for the structure into consideration. The selectivity of such a filter is essentially determined by the effective acoustic length of the reflectors.
Filter structures are described in European reference EP-A-0 032 313 corresponding to U.S. Pat. No. 4,319,154) which have in each case reduced temperature sensitivity despite the angled acoustic path. Provided there for this purpose in each case is a trio of reflectors with alteration of the angle of the acoustic path. It is important for there filter structures that the angle of incidence and the angle of reflection of the wave are the same for the middle reflector of this trio. There are no requirements with respect to the magnitude of the respective change of direction, which are at least approximately, 90.degree..
Embodiments of multi-track reflector filters with multi-strip couplers are stated in IEEE Trans. Ultrasonics, Vol. 35 (1988), pages 61 ff. A further embodiment stated therein provides for a filter having input transducer, output transducer and two reflector structures which are arranged in-line with one another. There are two mutually independent paths of the acoustic wave between the input transducer and the output transducer. Inserted in each of these paths is only one reflector for acute-angled reflection of the one wave in each case. Temperature compensation of the reflection angle is not provided.